Now I'm racking my brains and going back a looooong way here, but I seem to remember that the centre or star point of a 3 phase star arrangement is effectively the earth of the system. If the Aux terminal is never used then it may be that the DC electrical systems use the airframe itself as the return current route rather than add extra weight by wiring everything back to the Aux point. I could be way off base here (I'm sure you'll find a better answer in the Private Flying or engineers forums) but its been quite some time since I studied aircraft electrics and even longer since I did advanced electrical engineering!